Metabolism of trans-3-hexadecenoic acid by Chlorella vulgaris and by lettuce leaf

Biosynthesis of the unique trans-delta 3-hexadecenoic acid component of chloroplast phosphatidylglycerol: evidence concerning its site and mechanism of formation

As in most higher plants, chloroplast membranes of the green alga Dunaliella salina contain phosphatidylglycerol (PG) that is rich in trans-delta 3-hexadecenoic acid (16:1t), a fatty acid found nowhere else in the cell. After labeling D. salina with exogenous [3H]myristic acid [( 3H]14:0), the cis-unsaturated fatty acids of monogalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol as well as PG had higher specific radioactivities in chloroplast envelopes than in thylakoids. In contrast, 16:1t was very slow to become radioactive, and its specific radioactivity was several times higher in isolated thylakoids than in envelopes after brief (3-20 min) labeling with [3H]14:0. Analysis of individual PG molecular species revealed that the fatty acid paired with 16:1t was also labeled slowly. Thus linoleate (18:2) released from a 16:1t-containing PG had a 350-fold (at 3 min) to 20-fold (at 60 min) lower specific radioactivity than did 18:2 from a palmitate (16:0)-containing PG. The findings suggest that the substrates for trans-desaturation are 16:0-containing PG molecular species which are readily labeled from [3H]14:0 in the envelope but are diluted by the large pool of thylakoid PG before penetrating to the desaturation site. By examining the labeling patterns of individual PG molecular species classes, it was concluded that D. salina 16:1t is formed from 16:0 linked to 18:2/16:0 PG and 18:3/16:0 PG by a trans-desaturase located within the inner recesses of the thylakoid compartment.

Low-temperature-induced changes in intracellular fatty acid fluxes in Dunaliella salina

Two-minute exposures to exogenous [14C]palmitic, [14C]oleic, or [14C]lauric acid differentially labeled the lipids of Dunaliella salina microsomes and chloroplasts. Changes in fatty acid desaturation and intracellular movement during a subsequent 16-h incubation in nonradioactive medium indicated a slow transfer of lipids into the chloroplast from other organelles. Since Dunaliella lacks the massive traffic of microsomally produced glycerolipids into chloroplast galactolipids that dominates chloroplast-microsome lipid relations in most plant cells, it affords a sensitive system for studying more subtle intracellular lipid fluxes. Lowering the culture temperature from 30 to 12 degrees C was more inhibitory toward glycerolipid biosynthesis in chloroplasts than in microsomes. The ability of Dunaliella chloroplasts to utilize microsomal lipids may be essential for their systematic acclimation to low temperature.

The role of chloroplasts and microsomal fractions in polar-lipid synthesis from [1-14C]acetate by cell-free preparations from spinach (Spinacia oleracea) leaves

1. Isolated spinach (Spinacia oleracea) chloroplasts were incapable of accumulating polar lipids when incubated with [1-14C]acetate in a cofactor-free medium. When CoA, ATP and glycerol 3-phosphate were added to incubation media, the accumulated products were non-esterified fatty acids, acyl-CoA and 1,2-diacylglycerol, all intermediates of lipid metabolism. 2. Chloroplast acyl-CoA was used to synthesize phosphatidylcholine only when a microsomal fraction was added back to the incubation medium. 3. The 1,2-diacylglycerol synthesized by isolated chloroplasts was converted almost quantitatively into diacylgalactosylglycerol when exogenous UDP-galactose was available. 4. Stereospecific analyses of the isolated lipids suggested that the diacylglycerol synthesized by isolated chloroplasts may be an important precursor for the synthesis in vivo of diacylgalactosylglycerol and phosphatidylglycerol but was unlikely to be a precursor of phosphatidylcholine. 5. A scheme for plant-lipid biosynthesis is presented that integrates the functions of chloroplasts, the cytoplasm and the endoplasmic reticulum.

The kinetics of incorporation in vivo of (14C)acetate and (14C)carbon dioxide into the fatty acids of glycerolipids in developing leaves

1. The patterns of incorporation of (14)C into glycerolipid fatty acids of developing maize leaf lamina from supplied [1-(14)C]acetate and from (14)CO(2) during steady-state photosynthesis were similar. Oleate of phosphatidylcholine and palmitate of phosphatidylglycerol attained linear rates of labelling more rapidly than did other fatty acids, particularly the linoleate and linolenate of monogalactosyl diacylglycerol. 2. After the transfer of lamina from labelled to unlabelled acetate, there was a decrease in labelled oleate and linoleate of phosphatidylcholine and a concomitant increase in the amount of radioactivity in the linoleate and linolenate of monogalactosyl diacylglycerol. 3. The rapidly labelled phospholipids, phosphatidylcholine and phosphatidylglycerol, were shown by differential and sucrose-density-gradient centrifugation to be associated with different organelles, the former being mainly in a low-density membrane fraction, probably microsomal, and the latter mainly in chloroplasts. 4. During a 48h period after supplying spinach leaves with [(14)C]acetate, radioactivity was lost from the oleate of phosphatidylcholine present in fractions sedimented at 12000g and 105000g, and accumulated in the linolenate of monogalactosyl diacylglycerol of the chloroplast. 5. It is proposed that the phosphatidylcholine of some non-plastid membranes is intimately involved in the process of oleate desaturation and that this lipid serves as a donor of unsaturated C(18) fatty acids to other lipids, principally monogalactosyl diacylglycerol, of the chloroplasts.

Phosphatidyl choline: Donor of 18-carbon unsaturated fatty acids for glycerolipid biosynthesis

Kinetics of radiocarbon inforporation into the phyosphatidyl choline of pumpkin leaf fed 1(-14)C-acetate at low light intensity were strongly suggestive of lipid bound fatty acids acting as substrates for desaturase enzymes. After pulse labeling in direct sunlight with the same precursor, phosphatidyl choline and phosphatidyl glycerol contained up to 90% of total glycerolipid radiocarbon at the shortest sampling times. Subsequent loss of radiocarbon from phyosphatidyl choline and a corresponding gain in other glycerolipids is taken to indicate a net flow of long chain fatty acids through phosphatidyl choline and into other lipids. It is proposed that there may be 2 separate synthetases in leaf tissue, one producing oleic and the other palmitic acids as their end products. Oleic acid is transferred almost exclusively to phosphatidyl choline where it is further desaturated to linoleic and linolenic acids before being made available for the biosynthesis of other lipids. Palmitic acid is transferred mainly to phosphatidyl glycerol, where it is desaturated to trans-delta3-hexadecenoic acid.

Metabolism of trans-3-hexadecenoic acid in broad bean

1. Broad bean (Vicia faba) leaves contain rather high concentrations (about 4% of total fatty acids) of the trans-3-hexadecenoic acid. 2. Amounts of the acid increase with the age of the leaves and are absent from etiolated tissue. 3. Changes in the levels of trans-delta-4-hexadecenoic acid can be produced by subjecting the intact plants to various light/dark periods. 4. Chloroplasts isolated from broad-bean leaves show high rates of fatty acid synthesis from [1-14C]acetate. Synthesis is dependent on coenzyme A and ATP but is insensitive to the addition of exogenous acyl carrier protein. 5. The pattern of acids made includes about 20% palmitic, 5% hexadeconoic, 10% stearic and 60% oleic. trans-3-Hexadecenoic acid synthesis was most active in chloroplasts from plants exposed to the dark for 5 days and light for 3 days. 6. Arsenite addition inhibited stearate formation by isolated chloroplasts but resulted in a two-fold stimulation of overall synthesis. 7. The rate of fatty acid synthesis by isolated chloroplasts paralleled the changes in endogenous trans-3-hexadecenoic acid levels in the leaves from which they were isolated.

The inhibition of unsaturated fatty acid biosynthesis in plants by sterculic acid

Sterculic acid, 8‐(2‐octyl‐1‐cyclopropenyl)octanoic acid, is a potent inhibitor of the enzyme system converting stearic acid to oleic acid in Chlorella vulgaris. No inhibition of oleic acid for mation from acetate, is, however, observed. With the C10, C12 or C14 acids as precursor, inhibition increases with chain length. The enzyme system converting oleic acid to linoleic acid is also less sensitive than the stearate‐oleate system. Stearic acid generated internally in leaf preparations anaerobically from acetate is still converted to oleic acid on transference to air in the presence of sterculic acid. Desaturation of added palmitic acid is readily inhibited by sterculic acid, so far as formation of the Δ7 and Δ9 acids are concerned. Desaturation of palmitic acid to the 3‐trans‐hexadecenoic acid is unaffected by sterculic acid.

Two possible schemes to account for the effects of sterculic acid on stearate desaturation are suggested.